Aluminum oxide is known as a material which is relatively inexpensive and has excellent abrasion resistance. Also known is a technique of further improving mechanical properties such as the abrasion resistance or the strength of aluminum oxide by incorporating second phase such as tungsten carbide, as disclosed in JP-B-43-7719, JP-B-56-24704, JP-B63-17789, JP-B-64-1430, JP-A-52-52910, JP-A-57-205368, JP-A-62-59567, JP-A-62-153159, JP-A-63-69749 and JP-A-3-290355 ("JP-B" and "JP-A" disclosed herein mean examined and unexamined Japanese patent publication, respectively).
However, materials obtained by merely incorporating tungsten carbide into aluminum oxide as in the above technique are defective in that the grain diameters thereof are so large that grains may fall off the materials during abrasion. In addition, the incorporation of tungsten carbide in an amount of 5 wt % or less does not produce a sufficient effect, but the incorporation of tungsten carbide in amounts of 5 wt % or more result only in impaired mechanical strength.
Although the cause of the above has not been elucidated, it is presumed, from investigations conducted by the present inventors, that the insufficient or adverse effect of the incorporation of tungsten carbide is attributable to weak interfacial bonding between the aluminum oxide and tungsten carbide and to insufficient size reduction for aluminum oxide particles.